I think the flaw in that argument is the GPH per HP for a specific regime (ROP versus LOP) as your specific fuel consumption will change depending on that setting.  Your HP should be directly proportional to RPM and MP at any mixture. I think I'm correct in that last statement regardless of whether the engine is ROP or LOP.  When I ran the TSIO-520NB with GAMI the procedure would be to bump up the MP to keep the desired HP (65%, 70% or whatever you choose).   You have to bring the MP back up as the fuel flow drops off.    So, I would pick your desired power setting for LOP ops (say 65% for talking purposes) and get the MP and RPM for that power setting.   When you go LOP work towards bringing the MP back up to maintain your target power.  That is what I recall for the GAMIs so I would be interested if that might be what was being taught more recently.  As I said in an earlier posting, the only problem with high altitude turbo ops LOP was the need to bump up the MP to keep the wastegate from bootstrapping.  Others may need higher MP for pressurization but I never experienced that in the twin.  Hope that all makes sense.